3Lawrence J D.Hog marketing practices and competition questions [J] .Choices,2010(25).
4U.S.Deparlment of Agriculture,National Agricultural Statislics Service (USDA, NASS) .Hogs and pigs [R] .December 1991,1992,1996,2001,2012.
5U.S.Department of Agricuhure,National Agricul rural Statistics Service (USDA,NASS) .Farms, land in farms and livestock operations [R] .2006-2013.
6Key N.and McBride W.The changing economics of U.S.hog production [R] . USDA Economic Research Report Number 52,2007.
7U.S.Department of Agriculture,National Agricuhu ral Statistics Service (USDA,NASS).Livestock slaughter 2012 summary ER] .2013.
8U.S.Department of Agriculture,National Agricultural Statistics Service (USDA,NASS) .Livestock slaughter 1990 summary [R] .1991.
9MacDonald J M,McBride W D.The transformation of US livestock agriculture: Scale, efficiency, and risks [R] . US Department of Agriculture, Economic Research Service, 2009.
